Field of the Invention
The present invention relates to a magnetic resonance measurement apparatus, and in particular to a suppression technique for a transmission signal.
Description of Related Art
As magnetic resonance measurement apparatuses, nuclear magnetic resonance (NMR) measurement apparatuses and electron spin resonance (ESR) measurement apparatuses are known. In addition, as apparatuses classified as NMR measurement apparatuses, magnetic resonance imaging (MRI) apparatuses are also known. In the following, NMR measurement apparatuses will be described.
NMR refers to a phenomenon where an atomic nucleus under a static magnetic field interacts with an electromagnetic wave having a frequency intrinsic to the atomic nucleus. An apparatus that executes measurement of a sample at an atomic level taking advantage of this phenomenon is an NMR measurement apparatus. Currently, NMR measurement apparatuses are used in analyses of organic compounds (for example, medicines and agricultural chemicals), polymer materials (for example, vinyl and polyethylene), biological substances (for example, nucleic acids and proteins), or the like. With the use of an NMR measurement apparatus, for example, a molecular structure of the sample can be revealed.
An NMR measurement apparatus generally includes a control computer, a radio frequency (RF) signal transmitter, an NMR signal detector (probe), a static magnetic field generator (superconductive magnet), an NMR signal receiver, and the like. In some cases, a part of these structures is called an NMR measurement apparatus. For example, a part of a spectrometer including the control computer, the RF signal transmitter, and the NMR signal receiver may be called an NMR measurement apparatus. In a typical NMR measurement, a high-frequency signal for NMR measurement (RF transmission signal) is generated in the transmitter, and the transmission signal is supplied to a transmission and reception coil in the probe. A resonance absorption phenomenon is caused in an observation nucleus in the sample due to an electromagnetic wave caused by the transmission signal. An NMR signal induced in the transmission and reception coil (RF reception signal) is then transmitted to the receiver, and a spectrum of the received signal is analyzed.
In the NMR measurement apparatus, in general, a plurality of signal generators which operate in parallel with each other are provided in order to realize measurement based on a complex pulse sequence. Each individual signal generator is constructed from, for example, a DDS (Direct Digital Synthesizer) or an NCO (Numerical Controlled Oscillator) (for example, refer to JP 2012-49585 A). Each individual signal generator has functions of an intensity (amplitude) modulation, a phase modulation, a frequency modulation, or the like, and generates a transmission signal (digital signal) having a designated frequency. Each of a plurality of transmission signals generated by the plurality of signal generators is converted into an analog signal and then the plurality of signals are combined. With this process, an analog combined signal is generated. The analog combined signal is amplified by a power amplifier, and is transmitted to the probe as an RF transmission signal.
Here, a scheme may be considered in which, after the plurality of transmission signals are combined as digital signals, the digital combined signal thus generated is converted into an analog combined signal. According to such a configuration, the number of D/A (digital-to-analog) converters may be reduced, and signal processor circuits or the like provided downstream of each D/A converter can be omitted. However, in this case, if a necessary amplitude ratio (amplitude ratio to be finally achieved) is given for the plurality of transmission signals from the start in the generation stage of the plurality of transmission signals, there is a problem in that, for example, as a result of a pulse having a low amplitude which gradually changes as an analog waveform being expressed in the digital waveform with a small number of bits, the waveform of the pulse cannot be expressed in a smooth manner, and the waveform becomes a step-shaped waveform. With such a transmission signal, a further problem may be caused in that an output resolution of the DAC (D/A converter) cannot be fully utilized. Such a problem also arises in other magnetic resonance measurement apparatuses.
In an MRI apparatus disclosed in JP H8-289880 A, a variable attenuator is provided downstream of a switching amplifier. However, because the structure employs a switching amplifier which is not a linear amplifier, the amplitude of the transmission signal which is uniformly and greatly amplified at the amplifier is merely modulated by the variable attenuator after the amplification. An idea of realizing a proper amplitude ratio between a plurality of transmission signals after the synthesis or a structure therefor is not disclosed in JP H8-289880 A described above. This is also true for an apparatus disclosed in JP H5-23317 A.